There is a growing need to accommodate watermarks in audio and video signals. Watermarks are supplemental data messages embedded in multimedia assets, preferably in a perceptually invisible manner. They comprise information, for example, about the source or copyright status of documents and audiovisual programs. They can be used to provide legal proof of the copyright owner, and allow tracing of piracy and support the protection of intellectual property.
A known method of watermarking a video signal as defined in the opening paragraph is disclosed in F.Hartung and B.Giros: "Digital Watermarking of Raw and Compressed Video", SPIE Vol. 2952, pp. 205-212. Watermarking is herein achieved by modifying selected DCT coefficients in the output bitstream of an MPEG2 encoder, including DCT coefficients of predictively encoded (P or B) pictures. Predictive encoders include a feedback loop to control the encoding process. A prediction error (the difference between the input signal and a prediction therefor) is encoded rather than the input signal itself. The prediction signal is obtained by locally decoding the encoded signal.
In the prior art method, the watermarks are inserted after conventional coding. The capacity available for watermarking the encoded signal in this manner appears to be restricted.